1. Field of the Invention:
The present invention relates generally to integrated electronic circuits, and more specifically to a method for fabricating local interconnect on integrated circuits.
2. Description of the Prior Art:
The use of local interconnect technology is important for the fabrication of high density integrated circuits. As used herein, local interconnect will be used to refer generally to signal lines used to connect conducting regions which are more or less physically adjacent. Local interconnect is laid down and patterned without an intervening layer of oxide or other insulator through which contact vias must be formed.
High density memory and gate array integrated circuits can make advantageous use of local interconnect technology. For example, local interconnect can be used in a 6 transistor SRAM cell to reduce cell size. An example of the use of local interconnect for this purpose is shown in the paper VLSI LOCAL INTERCONNECT LEVEL USING TITANIUM NITRIDE, T. Tang et al, proceedings of the IEDM 1985, pages 590-593. The process described in such paper uses titanium nitride, which is formed as a by-product of other process steps, as a local interconnect in a 6 transistor SRAM cell.
U.S. Pat. No. 4,804,636 illustrates a similar use of titanium nitride as a local interconnect for VLSI MOS integrated circuits. In this patent, a layer of titanium nitride which is formed during a process step for forming titanium disilicide is used for local interconnect and contact pads. This patent illustrates the use of local interconnect in an SRAM cell.
Several properties of the material used for local interconnect in integrated circuits are important to overall functionality of the completed circuit. These include stability of the interconnect material during subsequent thermal cycles, and the long term integrity of the material. The material used must be stable and have long term integrity for use in high performance and reliable integrated circuit parts.
In the references described above, titanium nitride was used primarily because titanium forms a silicide over polycrystalline or monocrystalline silicon, and they can be etched selectively. This allows removal of selected titanium nitride regions without the potential of adversely impacting silicide regions formed out of the same titanium layer. However, the long term integrity, and stability during subsequent thermal cycles, of the titanium nitride used for local interconnect is not as good as desired for reliable integrated circuits.
It would be desirable to provide a method for forming local interconnect for integrated circuits which can be used to form such interconnect from a material which has long term stability and stability during subsequent thermal cycles.